The present invention relates to rotating electric machines such as synchronous machines, as well as dual-fed machines, applications in asynchronous static current converter cascades, outerpole machines and synchronous flow machines, and also alternating current machines intended in the first place as generators in a power station for generating electric power. The invention relates particularly to the stator of such machines concerning an embodiment for cooling stator teeth and thus indirectly also to the insulated electric conductors constituting the stator winding.
Similar machines have conventionally been designed for voltages in the range 6-30 kV, and 30 kV has normally been considered to be an upper limit. This generally means that a generator must be connected to the power network via a transformer which steps up the voltage to the level of the power network, i.e. in the range of approximately 130-400 kV. The machine is intended for use with high voltages. High voltages shall be understood here to mean electric voltages in excess of 10 kV. A typical operating range for a device according to the invention may be voltages from 36 kV up to 800 kV. In the second place the invention is intended for use in the stated technical area with voltages below 36 kV.
Two different systems exist for air cooling in conventional cooling: Radial cooling where the air passes the rotor through the hub and radial ducts in the rotor, and axial cooling where the air is blow into pole gaps by axial fans. The stator is then divided into radial air ducts by means of (usually straight) spacers which are welded in place. In view of the poor thermal conductivity axially through the stator laminations the air ducts must be frequently repeated. The drawback of air cooling is that the ventilation losses are often considerable and that the stator must be made longer to accommodate the ventilation ducts. The ventilation ducts may also cause a weaker mechanical structure, particularly in the case of the high-voltage generators with long teeth under discussion here.
Axial water cooling by means of cooling tubes in the stator yoke is well known. Electrically insulated metal tubes have then necessarily been used so as not to short-circuit the laminations of the stator. The drawback is that if the insulation is damaged the generator may be destroyed by the induced currents then appearing. It is also expensive to weld or bend the tubes at the joins. Another drawback is that eddy currents are induced in metal tubes in a time-varying magnetic flow, resulting in certain power losses when they are used in an electric machine.
A conductor is known through U.S. Pat. No. 5,036,165, in which the insulation is provided with an inner and an outer layer of semiconducting pyrolized glassfiber. It is also known to provide conductors in a dynamo-electric machine with such an insulation, as described in U.S. Pat. No. 5,066,881 for instance, where a semiconducting pyrolized glassfiber layer is in contact with the two parallel rods forming the conductor, and the insulation in the stator slots is surrounded by an outer layer of semiconducting pyrolized glassfiber. The pyrolized glassfiber material is described as suitable since it retains its resistivity even after the impregnation treatment.
By using high-voltage insulated electric conductors, in the following termed high-voltage cables, with solid insulation similar to that used in cables for transmitting electric power in the stator winding (e.g. XLPE cables) the voltage of the machine can be increased to such levels that it can be connected directly to the power network without an intermediate transformer. The conventional transformer can thus be eliminated. This concept generally requires the slots in which the cables are placed in the stator to be deeper than with conventional technology (thicker insulation due to higher voltage and more turns in the winding). This entails new problems with regard to cooling, vibrations and natural frequencies in the region of the coil end, teeth and winding.
The object of the invention is to provide a stator in a rotating electric machine with an end plate for use in direct cooling of the stator, particularly the stator teeth in a rotating electric machine of the type described, said cooling being achieved by means of cooling tubes running axially in the stator. The purpose of the stator plate is to provide protection for the cooling tubes at the ends of the stator. The cooling tubes are exposed to mechanical stress at each end of the stator during assembly, which is eliminated through the present invention.
Another object of the invention is for the stator plate to constitute a bending template for the cooling tubes during assembly. Additional advantageous further developments of the invention are indicated in the following description. The invention is in the first place intended to be used with a high-voltage cable defined in more detail below, and its advantages are particularly noticeable therewith.
The present invention relates to a stator end plate in connection with axial cooling of the stator and its laminated stack, particularly the stator teeth, and thus indirectly the stator winding in a rotating electric machine such as a high-voltage alternating current generator.
The plate is provided with axially running winding slots corresponding to the stator, and axially running apertures for inlet and outlet cooling tubes. The plate is also provided with slits in which bending members are situated, around which bending members the cooling tubes are arranged to be bent.
The end plate is also provided with assembly grooves intended to retain sealing member at the exit of each winding slot from the end plate.
In the device according to the invention the windings are preferably composed of cables having solid, extruded insulation, of a type now used for power distribution, such as XLPE-cables or cables with EPR-insulation. Such a cable comprises an inner conductor composed of one or more strand parts, an inner semiconducting layer surrounding the conductor, a solid insulating layer surrounding this and an outer semiconducting layer surrounding the insulating layer. Such cables are flexible, which is an important property in this context since the technology for the device according to the invention is based primarily on winding systems in which the winding is formed from cable which is bent during assembly. The flexibility of a XLPE-cable normally corresponds to a radius of curvature of approximately 20 cm for a cable 30 mm in diameter, and a radius of curvature of approximately 65 cm for a cable 80 mm in diameter. In the present application the term xe2x80x9cflexiblexe2x80x9d is used to indicate that the winding is flexible down to a radius of curvature in the order of four times the cable diameter, preferably eight to twelve times the cable diameter.
The winding should be constructed to retain its properties even when it is bent and when it is subjected to thermal stress during operation. It is vital that the layers retain their adhesion to each other in this context. The material properties of the layers are decisive here, particularly their elasticity and relative coefficients of thermal expansion. In a XLPE-cable, for instance, the insulating layer consists of cross-linked, low-density polyethylene, and the semiconducting layers consist of polyethylene with soot and metal particles mixed in. Changes in volume as a result of temperature fluctuations are completely absorbed as changes in radius in the cable and, thanks to the comparatively slight difference between the coefficients of thermal expansion in the layers in relation to the elasticity of these materials, the radial expansion can take place without the adhesion between the layers being lost.
The material combinations stated above should be considered only as examples. Other combinations fulfilling the conditions specified and also the condition of being semiconducting, i.e. having resistivity within the range of 10xe2x80x941-106 ohm-cm, e.g. 1-500 ohm-cm, or 10-200 ohm-cm, naturally also fall within the scope of the invention.
The insulating layer may consist, for example, of a solid thermoplastic material such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP) polybutylene (PB), polymethyl pentene (PMP), cross-linked materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene propylene rubber (EPR) or silicon rubber.
The inner and outer semiconducting layers may be of the same basic material but with particles of conducting material such as soot or metal powder mixed in.
The mechanical properties of these materials, particularly their coefficients of thermal expansion, are affected relatively little by whether soot or metal powder is mixed in or notxe2x80x94at least in the proportions required to achieve the conductivity necessary according to the invention. The insulating layer and the semiconducting layers thus have substantially the same coefficients of thermal expansion.
Ethylene-vinyl-acetate copolymers/nitrile rubber, butyl graft polyethylene, ethylene-butyl-acrylate-copolymers and ethylene-ethyl-acrylate copolymers may also constitute suitable polymers for the semiconducting layers.
Even when different types of material are used as base in the various layers, it is desirable for their coefficients of thermal expansion to be substantially the same. This is the case with combination of the materials listed above.
The materials listed above have relatively good elasticity, with an E-modulus of E less than 500 MPa, preferably  less than 200 MPa. The elasticity is sufficient for any minor differences between the coefficients of thermal expansion for the materials in the layers to be absorbed in the radial direction of the elasticity so that no cracks appear, or any other damage, and so that the layers are not released from each other. The material in the layers is elastic, and the adhesion between the layers is at least of the same magnitude as the weakest of the materials.
The conductivity of the two semiconducting layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outer semiconducting layer is sufficiently large to enclose the electrical field in the cable, but sufficiently small not to give rise to significant losses due to currents induced in the longitudinal direction of the layer.
Thus, each of the two semiconducting layers essentially constitutes one equipotential surface, and these layers will substantially enclose the electrical field between them.
There is, of course, nothing to prevent one or more additional semiconducting layers being arranged in the insulating layer.